There are frequency bands in which different kinds of wireless communication systems can coexist. One example of the frequency bands is a TV white space. The TV white space is an idle frequency band reserved due to digitalization of an analog broadcasting. The TV white space is a spectrum of 512 to 698 MHz allocated to a broadcast TV. When a licensed device is not used in the corresponding spectrum, an unlicensed device can use the corresponding band.
When IEEE 802.11 is used in the TV white space, there is an advantage in that the coverage is remarkably expanded due to the spectrum's characteristic. However, generally, as the coverage is expanded, the number of stations considerably increases. In this situation, a problem may arise in a capability of flexibly handling users with increase of the number of the users, i.e. scalability. Moreover, since several wireless communication systems coexist and several unlicensed devices coexist, a problem may arise in coexistence. If a Distributed Coordination Function (DCF) and an Enhanced Distributed Channel Access (EDCA) protocol of IEEE 802.11 are applied in this environment, scalability may be more deteriorated.
The DCF is a channel access mechanism used in IEEE 802.11 and is based on Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). Also, the EDCA is equivalent to a competition-based medium access method among channel access models suggested by a Hybrid Coordination Function (HCF) defined by expanding a general medium access control protocol of IEEE 802.11. Here, the HCF is a protocol defined in IEEE 802.11e suggested to ensure Quality of Service (QoS).
Further, when the protocol of IEEE 802.11 is applied in the TV white space, it is expected that overlapping Basic Service Sets (BSSs) will increase sharply. For example, if a user arbitrarily installs an Access Point (AP) supporting the TV white space, since the coverage of the AP is wide, an overlapping service area may increase.
When the unlicensed device intends to use the TV white space, it should acquire an available channel in a corresponding area, using a geo-location database. In addition, a signaling protocol or a spectrum sensing mechanism is necessary to solve a problem in coexistence of the unlicensed devices using the TV white space.